You can choose between steel, solid brass, iron and copper plated variations. Most commonly used are 1.1 and 1.2mm medium-thin lasting nails because they represent the best ratio between thickness and strength. For thick leather, you can use 1.4mm thick nails.
If you work with light color leather, consider using copper plated nails to avoid a dark stain on leather.
All nails are made from carefully selected and inspected materials so they are not too soft but yet not too hard, so perfect for shoe leather lasting.